The present invention relates to a process for the preparation of N-alkylated anilines by reaction of anilines with lower alcohols or dialkyl ethers at elevated temperature and in the presence of proton-containing zeolites of the pentasil type having an SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least 60.
N-alkylated anilines are important industrial intermediates for the preparation of dyestuffs, stabilizers, urethanes, ureas, pharmaceuticals and plant-protection agents. They are usually prepared by alkylation of anilines with alcohols in the presence of acidic catalysts, for example phosphorus oxychloride, under pressure or by passing aniline and alcohol vapors together through hot phosphoric acid at atmospheric pressure.
These processes are unsatisfactory in some respects. The reaction in an autoclave under pressure is industrially expensive, and the homogeneously dissolved acidic catalysts cause extensive corrosion. The phosphoric acid process at atmospheric pressure has this disadvantage to a lesser extent, the relatively large amount of phosphoric acid becomes unusable as a catalyst over time and has to be disposed of and replaced by fresh phosphoric acid.
It is true of both processes that they have only relatively little flexibility with respect to the alternating preparation of N-monoalkylanilines and N,N-dialkylanilines.
Zeolites have already been proposed as catalysts for these reactions, and it has been shown that both N-monoalkyl- and also N,N-dialkylanilines can be obtained. However, other disadvantages arise.
According to Waseda Daigaku Rikogaku Konkyusho Hokoku 69 (1975), 21-25 (cited in C.A. 84 (1976), 121 339 k), Y-zeolites and also H-Y as well as those exchanged with Cu, Ni, Co, Mn, Zn, Ca and Ce ions are suitable as catalysts for the reaction of methanol with aniline to give N-methylated anilines. However, from Table 1 on page 23 of this reference, it can be seen that all reactions are accompanied by the formation of toluidine, that is, by ring-alkylation. The highest activity (100 % of conversion) and one of the highest rates (86%) of N-methylation is shown by H-Y, however only at an uneconomical molar ratio of methanol: aniline=3:1. The highest N-methylation rate (92 %) is shown by Cu-Y, in which case, however, the conversion is significantly less (44.9%) and the formation of toluidine is still 3.4%.
In addition, the reaction on Cu-Y and H-Y has a very unfavorable temperature dependence, since a useful conversion is achieved only in a very narrow temperature range around 250.degree. C. (cf. FIGS. 1 and 2 on page 22), whereas at other only slightly changed temperatures not only conversion but also yield in N-alkylated anilines decrease.
P. Y. Chen, M. C. Chen, H. Y. Chu, N. S. Chang and T. K. Chuang, Proc. 7th Intern. Zeolite Conf. Y. Murakami, A. Iijima and J. W Ward (Eds.), p. 739-744, Kodansha, Tokyo and Elsevier, Amsterdam, Oxford, New York, Tokyo 1986 show in their investigation of N-methylation of aniline with methanol on ZSM-5 zeolites that ring-alkylation is always observed and they are convinced that it is caused by active centers on the surface of the zeolites. Furthermore, not only the basic but also the acidic properties of the zeolites are said to have been responsible for catalytic activity in such a manner that upon impregnation of the zeolites with metal oxides ring-alkylation decreases. Yet, even under the most favorable conditions (table on p. 744) using MgO/H-ZSM 5, noticeable ring-alkylation is still found. In addition, a temperature of 350.degree. C. and a large excess of methanol must be used. This indicates a very low activity of these zeolites. Furthermore, an increase in SiO.sub.2 /Al.sub.2 O.sub.3 ratio results in decreasing conversion of aniline (FIG. 1 on p. 741). Admittedly, the selectivity increases but even in the most favorable case ring-alkylation still amounts to several % (FIG. 2 on p. 742). Consequently, the acidic H-ZSM 5 appears to be the least suitable zeolite. It almost produces the lowest aniline conversion and is rapidly deactivated (FIG. 3 on p. 743) and furthermore causes one of the highest ring-alkylation rates (Table 2 on p. 743).